The present invention relates to a control apparatus for electric motor vehicles and in particular to a control apparatus which assures starting of the electric motor vehicle even on an uphill gradient with an improved reliability and safety.
The hitherto known electric motor vehicles incorporating a battery as a power supply source such as a fork lift truck for example may be generally classified into two types, i.e. a first type in which an accelerator pedal and a brake pedal for actuating a hydraulic-mechanical brake system are laterally separately provided with an appropriate distance therebetween so as to be manipulated individually by the feet of an operator as is common in conventional motorcars and a second type in which both of the pedals are disposed at one lateral side so as to by operated by a single foot of the operator. In conventional electric motor vehicles, a counter current braking system or plugging system has been usually utilized. In the case of the first type motor vehicles, a so-called uphill starting (starting of a motor vehicle on the way of an uphill gradient road) can be effected safely and smoothly without giving rise to the danger of spontaneous reverse movement of the vehicle under gravity by virtue of the fact that the brake pedal and the accelerator pedal can be manipulated individually by two feet, as it is common in the conventional motorcars. On the other hand, in the case of the second type motor vehicles, the vehicle will be likely to be moved backward spontaneously upon the uphill starting, because the brake pedal has to be released before the accelerator pedal can be actuated. In the electric motor vehicles having the plugging mechanism, once the spontaneous reverse or backward movement has occured upon the uphill starting, it becomes difficult to suppress such retrogression and restart the vehicle in the forward direction, even if the accelerator pedal is actuated to its full stroke. Because once the spontaneous reverse or backward movement has occured upon the uphill starting, a plugging circuit is formed, but the plugging force is comparatively weak.
Next, referring to FIG. 1, description in detail will be made on a hitherto known control apparatus utilizing a chopper circuit feeding system for a battery fork lift truck provided with a conventional counter current braking system or plugging means which is usually operative in the course of running on a flat surface. Referring to FIG. 1, an armature MA of a drive motor, a current detector SH for detecting the armature current, a switching contactor device S.sub.0 and a chopper circuit CH are connected in series to a battery Eb. The contactor device S.sub.0 is adapted to close contacts S.sub.11 and S.sub.12 for a forward movement, while closing contacts S.sub.21 and S.sub.22 for a backward drive thereby to reverse the polarity of field current flowing through the field winding MF of the motor. Connected in parallel to the series connection of the armature MA, the current detector SH and the contactor S.sub.0 is a plugging diode D.sub.p which is adapted to allow the motor current to flow therethrough upon plugging operation. A fly-wheel diode D.sub.F is connected in parallel to the series connection of the armature MA and the detector SH to conduct therethrough inductive energy of the armature MA and the field winding MF in response to the chopper circuit CH being turned off.
The control in the course of the forward running with the forward contactors S.sub.11 and S.sub.12 being closed is effected in the manner described below. The actuated position of the accelerator foot pedal 1 is converted into a corresponding electric quantity through a circuit command signal transducer 2, the output command signal from which is applied to a time delay circuit 3 having a predetermined time constant thereby to obtain an armature current command value V.sub.P. In dependence on the difference between the armature current command value V.sub.P and the detected current V.sub.I of the armature current I.sub.A, the phase angle of the output signal from a phase shifter 4 having also an amplifying function is controlled and applied to a pulse converter 5 which in turn produces "ON" and "OFF" signals S.sub.ON and S.sub.OFF to the chopper circuit CH. With such arrangement of the automatic current regulator ACR, the output signals S.sub.ON and S.sub.OFF of the chopper circuit CH is so controlled that the armature current I.sub.A may follow the current command value V.sub.P given by the actuation of the accelerator foot pedal 1, thereby to allow the forward power running of the vehicle through the constant current control. The control of the drive motor with the aid of a chopper circuit has been widely adopted in the prior art. Of course, the above described circuit arrangement has been hitherto known.
When the contactors S.sub.21 and S.sub.22 for the backward drive are closed during the forward power running of the electric motor vehicle, the rotation of the rotor or armature of the electric motor in the forward direction is still maintained at that instant. However, because the polarity of current flowing through the field system MF is inverted through the change-over of the contactor drive S.sub.0 as described above, a brake effort due to counter current braking is applied to the rotor. In this connection, it will be appreciated that the counter current braking effort will become greater proportionally as the current flowing through the field system is larger. Under such plugging condition, the polarity of the voltage induced in the motor is opposite to that of the voltage induced during the forward running and is the same as that of the battery voltage E.sub.b, whereby impedance of the motor circuit is equivalently reduced. As a consequence, the duty cycle of the chopper circuit CH is decreased to a small value (less than about 5%), because the armature current I.sub.A is caused to follow the current command value V.sub.P.
When the rotation of the drive motor in the forward running direction is stopped due to the plugging then the electric motor vehicle can start the backward running through the constant current control, provided that the contactors S.sub.21 and S.sub.22 for the backward running are maintained in the closed position.
It should be mentioned that the armature current command value V.sub.P corresponding to the magnitude of actuation of the accelerator foot pedal is so set that there can be produced the actual armature current I.sub.A which assures desired running performances of the vehicle in both the forward and the backward directions. The circuit shown in FIG. 1 is so arranged that the constant current control is performed in which the armature current I.sub.A is caused to be equal to or to follow the current command value V.sub.P even at the time of the plugging condition. In other words, the plugging is effected on the basis of the current command value V.sub.P which conform to the running performances of the vehicle in the forward and the backward directions. As a consequence, the plugging performance is determined definitely in dependence on the specification of the electric motor and impedance of the motor circuit as employed for a given value of the current command signal V.sub.P. Alternatively, it was impossible to attain an increased braking effort by correspondingly increasing the field current only at the plugging time.
Now, it is assumed that a battery motorcar such as fork lift truck is going to start on an uphill gradient road. When the accelerator pedal 1 is actuated after the vehicle has undergone the spontaneous retrogression thereby to produce the armature current I.sub.A in accordance with the current command value V.sub.P, plugging will occur in the electric motor because of the rotation of the rotor or armature in the backward or reverse direction. It is possible to start the vehicle even at the uphill gradient if the reverse rotation of the drive motor can be stopped through the plugging. However, it should be noted that such plugging performance is generally so selected as to be suited for the running on a flat road, as a result of which there after arises the case wherein the uphill starting of vehicle is rendered impossible in dependence on the gradient of the uphill road and the load on the electric motor vehicle because the available plugging turque is insufficient for preventing or stopping the spontaneous backward movement of the vehicle.